Implants are being employed in a wide variety of forms in modern medical technology. They are used, for example, to support vessels, hollow organs and vein systems (endovascular implants, such as stents), for fastening and the temporary fixation of tissue implants and tissue transplantations, but also for orthopedic purposes, such as nails, plates or screws. A particularly frequently used form of an implant is the stent.
The implantation of stents has become established as one of the most effective therapeutic measures for the treatment of vascular diseases. Stents have the purpose of performing a stabilizing function in hollow organs of a patient. For this purpose, stents featuring conventional designs have a filigree supporting structure comprising metal braces, which is initially present in compressed form for introduction into the body and is expanded at the site of the application. One of the main application areas of such stents is to permanently or temporarily dilate and hold open vascular constrictions, particularly constrictions (stenoses) of the coronary blood vessels. In addition, aneurysm stents are also known, which are used primarily to seal the aneurysm. The support function is additionally provided.
Stents comprise a peripheral wall with sufficient load-bearing capacity to hold the constricted vessel open to the desired extent and a tubular base body through which the blood continues to flow without impairment. The peripheral wall is generally formed by a lattice-like supporting structure, which allows the stent to be introduced in a compressed state, in which it has a small outside diameter, all the way to the stenosis of the particular vessel to be treated and to be expanded there, for example by way of a balloon catheter, so that the vessel has the desired, enlarged inside diameter. As an alternative, shape memory materials such as nitinol have the ability to self-expand when a restoring force is eliminated that keeps the implant at a small diameter. The restoring force is generally applied to the material by a protective tube.
The implant, notably the stent, has a base body made of an implant material. An implant material is a non-living material, which is used for applications in medicine and interacts with biological systems. A basic prerequisite for the use of a material as implant material, which is in contact with the body environment when used as intended, is the body friendliness thereof (biocompatibility). Biocompatibility shall be understood as the ability of a material to evoke an appropriate tissue response in a specific application. This includes an adaptation of the chemical, physical, biological, and morphological surface properties of an implant to the recipient's tissue with the aim of a clinically desired interaction. The biocompatibility of the implant material is also dependent on the temporal course of the response of the biosystem in which it is implanted. For example, irritations and inflammations occur in a relatively short time, which can lead to tissue changes. Depending on the properties of the implant material, biological systems thus react in different ways. According to the response of the biosystem, the implant materials can be divided into bioactive, bioinert and degradable or resorbable materials.
Implant materials comprise polymers, metallic materials, and ceramic materials (as coatings, for example). Biocompatible metals and metal alloys for permanent implants comprise, for example, stainless steels (such as 316L), cobalt-based alloys (such as CoCrMo cast alloys, CoCrMo forge alloys, CoCrWNi forge alloys and CoCrNiMo forge alloys), technical pure titanium and titanium alloys (such as cp titanium, TiAl6V4 or TiAl6Nb7) and gold alloys. In the field of biocorrodible stents, the use of magnesium or technical pure iron as well as biocorrodible base alloys of the elements magnesium, iron, zinc, molybdenum, and tungsten are proposed. Aspects of the present invention relate to biocorrodible magnesium base alloys.
The use of biocorrodible magnesium alloys for temporary implants having filigree structures is made difficult in particular in that the degradation of the implant progresses very quickly in vivo. So as to reduce the corrosion rate, this being the degradation speed, different approaches are being discussed. For one, it is attempted to slow the degradation on the part of the implant material by developing appropriate alloys. In addition, coatings are to bring about a temporary inhibition of the degradation. While the existing approaches are promising, none of them has so far been implemented in a commercially available product. Regardless of the efforts made so far, there is rather a continuing need for solutions that make it possible to at least temporarily reduce the in vivo corrosion of magnesium alloys.